The present invention relates generally to micro-electromechanical devices for use in optical systems. More particularly, the present invention relates to improved spectral equalizers and optical switches.
As wavelength-division-multiplexed (xe2x80x9cWDMxe2x80x9d) signals propagate through a fiber optic network, they lose strength. To compensate, the signals are amplified at various points in the network, using, for example, an Erbium-doped fiber amplifier.
Erbium-doped fiber amplifiers amplify each of the channels (i.e., wavelengths or spectral components) comprising a WDM transmission, but provide an uneven gain across the spectrum. In other words, some spectral components or wavelengths are amplified to a far greater extent than others. And this disparity in signal strength grows as a result of multiple cycles of losses and amplification. Uncorrected, the signal-strength disparity results in transmission errors. Consequently, a fiber optic network should incorporate a means for smoothing-out or equalizing such wavelength-to-wavelength disparities.
Early techniques for spectral equalization involved selectively pre-emphasizing the signal strength of certain channels, or using fixed wavelength-selective fiber gratings or using individual attenuators that are placed between wavelength routers. Each of these approaches disadvantageously impacts network flexibility.
Recent proposals for spectral equalization involve spatially resolving the plural spectral components of a WDM signal and delivering them to either (1) individual optical modulators (U.S. Pat. No. 5,745,271) or (2) a continuous modulator xe2x80x9cstripxe2x80x9d (U.S. Pat. No. 5,943,158). Both of these modulator configurations utilize an optical interference principle wherein the reflectivity of the modulator is varied by changing the size of an optical cavity. By appropriately varying the reflectivity of: (1) the individual optical modulators or (2) regions along the modulator strip, the signal strengths of the various reflected spectral components are equalized. U.S. Pat. Nos. 5,745,271 and 5,943,158 are both incorporated by reference herein.
Despite the considerable advantages of these latter proposals, they have several drawbacks. In particular, the nature of optical interference-type devices is such that the membrane or movable layer of the optical cavity has stringent thickness constraints. Furthermore, device performance is wavelength specific. Consequently, a continuing need exists in the art for equalization devices and techniques that enhance optical network performance.
An array of movable reflective elements provides wavelength independent performance in spectral equalizers and other optical devices in accordance with the principles of the present invention. Each movable reflective element comprises a reflective plate that is suspended by rod-like supports.
Typically, supports that are used in MEMS structures comprise serpentine-shape members. The serpentine configuration is used because it provides a soft and very compact support element and handles compressive and tensile stresses well. On the other hand, the serpentine configuration introduces mechanical softness for both in-plane and out-of-plane translational motion. Out-of-plane softness can disadvantageously decrease the angular range of motion. In fact, under certain conditions, this effect is so pronounced that serpentine-shape supports should not be used. Specifically, it has been discovered that when the supported structure (e.g., a movable plate, etc.) is comparable in size to the serpentine support, only about two thirds of the theoretical rotation attainable from an applied force (e.g., electrostatic, etc.) is observed. The balance of the applied force results in a sag in the serpentine support.
When such a condition exists, a rod-like support having an extraordinarily narrow width, at least in the context of MEMS devices, is advantageously used. The width of the rod-like supports described herein are, in some embodiments, less than about 0.5 microns. Since conventional MEMS fabrication procedures are not capable of producing a support having a width less than about 2 microns, a different approach must be used. In accordance with the principles of the invention, semiconductor processing techniques are used to provide a support having a width that is, in some embodiments, substantially less than 2 microns. The supports, referred to herein as xe2x80x9crod supportsxe2x80x9d (because of a straight, rod-like structure) are free to twist in torsional motion to allow the reflective plate that is linked thereto to tilt or rotate, with substantially no sagging, about an axis that is aligned with the rod supports.
A method in accordance with the present invention for making an array of movable reflective elements comprises the steps of: forming electrodes in a first substrate; forming a spacer on the first substrate; forming movable reflective elements in a second substrate using semiconductor processing techniques to provide a rod support that has a width that is less than 2 microns; and attaching the second substrate to the first substrate so that the movable reflective elements are proximal to the electrodes.
An improved spectral equalizer incorporates the array of movable reflective elements, a controller, collimating/focusing optics and a diffraction element that optically communicate in a free-space optics arrangement. In use, the collimating/focusing optics of the improved spectral equalizer receives a multiplexed optical signal from an optical fiber and collimates all wavelengths comprising the signal. The collimated multiplexed optical signal impinges upon the diffraction element. The diffraction element diffracts the signal such that the various wavelengths or spectral components of the multiplexed signal spatially separate as they move away from the diffraction element toward the collimating/focusing optics. The optics focus the spatially separated spectral components onto a different one of the movable reflective elements in the array of same, which is located at the Fourier plane (i.e., the back focal plane of the optics).
The movable reflective elements, which are independently movable about an axis of rotation, are directed to rotate a desired angle by the controller. After being reflected by the xe2x80x9ctiltedxe2x80x9d movable reflective elements, the various spectral components are collimated by the collimating/focusing optics and impinge again upon the diffraction element. The diffraction element re-multiplexes the spectral components and directs the re-multiplexed signal to the collimating/focusing optics, which then couples the signal to optical fiber. Since the various movable reflective elements are rotated or tilted, the various spectral components of the re-multiplexed signal are received xe2x80x9coff axisxe2x80x9d by the optical fiber. Any deviation from the optical axis reduces the intensity or power of a signal that is coupled back into the optical fiber. Therefore, the strength of each spectral component of the original multiplexed signal is individually attenuated to a desired degree to equalize power across the spectrum by selectively rotating the appropriate movable reflective elements.